In recent years, the introduction of Distributed Generators (DG), of which photovoltaic (PV) systems are representative, has been advanced due to environmental issues and the decreasing of dependence on fossil fuels.
Most distributed generators are linked to distribution power lines for distributing electric power that is supplied from distribution substations to industries, public facilities, private residences, and multiple-family dwellings (hereinbelow collectively referred to as “consumers”). For example, in a PV system, direct-current power that is generated by a photovoltaic panel is converted to an alternating current voltage appropriate for an electric power grid and supplied to distribution lines by means of a PCS (Power Conditioning System).
A distributed generator is usually independently managed by a relatively small-scale industry or user, and linking a multiplicity of such distributed generators to an electric power grid introduces the potential for disturbance of the electric power grid. The Agency for Natural Resources and Energy has therefore indicated guidelines for linking a distributed generator to an electric power grid (See Non-Patent Document 1).
One disturbance that occurs with the spread of distributed generators is the problem of voltage deviation. According to Non-Patent Document 1, distribution line voltage that is supplied to a low-voltage consumer must be maintained within the range of 101±6V for a standard voltage of 100V and must be maintained within a range of 202±20V for a standard voltage of 200V.
An electric power grid is made up by high-voltage distribution lines for transmitting high-voltage electric power that is supplied from the distribution transformer of an electric power substation and low-voltage distribution lines for transmitting low-voltage electric power that has been converted from high voltage to low voltage by pole transformers, each consumer typically being connected to a low-voltage distribution line. These distribution lines are connected in a radiating form so as not to form a loop, and in a conventional electric power grid in which only the load is connected, voltage is assumed to drop for the remote ends of the low-voltage distribution lines that are connected in a radiating form, and lines are therefore designed such that voltage does not deviate from the proper range even for the ends of the low-voltage distribution lines.
Nevertheless, the connection of a distributed generator to an electric power grid results in a decrease in the current that is supplied from the electric power grid to the consumer that is equipped with the distributed generator, whereby the distribution line voltage increases and thus deviates from the acceptable range. In addition, selling surplus electric power that has been generated by a distributed generator back to the electric power company results in the phenomenon called “reverse power flow” in which current flows from the consumer in the direction of the pole transformer, and this reverse power flow increases the potential for deviation from the acceptable range due to the increase in the distribution line voltage that increases with the remoteness on the low-voltage distribution line. The tendency for deviation from the acceptable range of the distribution line voltage differs according to location, the occurrence of voltage deviation becoming more likely with increasing remoteness on a low-voltage distribution line and the occurrence of voltage deviation on a high-voltage distribution line also becoming more likely with increasing remoteness. In addition, the distribution line voltage is in some cases originally set high by the turn ratio of pole transformers on a low-voltage distribution line, and distribution line voltage is therefore prone to deviation from the acceptable range on this type of low-voltage distribution line as well. Still further, when a high-volume distributed generator is linked to any point of a low-voltage distribution line or high-voltage distribution line, the distribution line voltage is prone to deviation from the acceptable range in the vicinity of the connection point.
To avoid this problem of voltage deviation, a distributed generator is required to be equipped with functions for autonomously suppressing voltage increase that are referred to as active power control (P control) and reactive power control (Q control).
According to Non-Patent Document 1, when the distribution line voltage at a connection point exceeds 107V, which is the upper limit of the acceptable range, a distributed generator is required to decrease the distribution line voltage by supplying a phase-advance reactive power (Q control) until the power factor falls to 85%, and if the distribution line voltage still does not return to within the acceptable range, is required to suppress the generated amount (P control).
However, because the suppression of voltage increase by means of Q control by a distributed generator alone has little effect, and moreover, because adding Q control capability to PCS is costly, many PCS lack Q control capability. In such cases, the distributed generator immediately initiates P control when the distribution line voltage exceeds the acceptable range at the connection point,
As described hereinabove, because the locations prone to the occurrence of voltage deviation such as the ends of low-voltage distribution lines are fixed, control for suppressing voltage increase (hereinbelow referred to as voltage suppression control) is concentrated among the portion of consumers for whom distributed generators are linked to such locations.
Voltage suppression control incurs losses for a consumer due to increase in costs arising from providing Q control capability in a PCS, the deterioration of PCS due to the use of P control function and Q control function, and decrease of the amount of electricity that is sold back due to limitation of the amount of generated power. The concentration of these losses among a portion of consumers raises problems, and these problems are exacerbated by the occurrence of inequalities among consumers who are relatively close, for example, connected to the same pole transformer and among consumers whose power generation conditions, such as weather, are identical.
One example of a method for solving the inequality of the concentration of losses to a portion of consumers while still avoiding the problem of voltage deviation is proposed in Non-Patent Document 2, this being a method of, when voltage deviation occurs, instructing the output of reactive power to all PCS linked to the high-voltage distribution line to which is connected the low-voltage distribution line in which the voltage deviation occurred.
However, in the method proposed in Non-Patent Document 2, because a PCS lacking Q control capability simply does not carry out voltage suppression control, consumers equipped with PCS that lack Q control capability and consumers equipped with PCS that have Q control capability are treated unequally. In addition, even when the amount of control (amount of voltage adjustment) for suppressing a voltage increase by any consumer is the same, the amount of change in the distribution line voltage at the connection points of other consumers typically differs according to the location of connection of distributed generators in which control is implemented. As a result, equal allocation of Q control to PCS that are linked at locations for which the effect of suppressing voltage increase is limited as proposed by Non-Patent Document 2 results in a great increase of the total amount of voltage adjustment for suppressing an increase in voltage, whereby an increase in voltage cannot be effectively suppressed. As described hereinabove, voltage suppression control incurs losses for consumers, and limiting to the utmost the amount of voltage adjustment to suppress a voltage increase is therefore to be desired. In addition, the suppression of a voltage increase by Q control typically incurs great power distribution loss, whereby loss suffered by electric power companies also increases. As a result, the total amount of Q control that results from each distributed generator for suppressing a voltage increase should be limited as much as possible.
According to one method that has been considered for solving the inequality of the concentration of losses to a portion of consumers and for further decreasing the total amount of voltage adjustment to suppress a voltage increase while still avoiding the problem of voltage deviation, the burden of large voltage adjustment amounts is placed on PCS that are linked at positions for which the effect of suppressing an increase in voltage is great, and losses resulting from the inequality of voltage adjustment amounts are compensated by fees. For example, Non-Patent Document 3 proposes the payment of an incentive to each consumer that is equipped with a distributed generator according to the amount of reactive power output from each distributed generator. This method, however, entails the problem that many issues, such as the deterioration of PCS caused by the use of P control function and Q control function, must be considered, and determining the incentives that can resolve inequalities is therefore problematic.
In the electric power grid control systems of the background art described hereinabove, the problem remains that inequality occurs due to concentrating voltage suppression control in a portion of the consumers when voltage suppression control is performed by distributed generators due to such issues as the locations of connection of distributed generators to the electric power grid and whether an owned PCS has Q control capability or not.
In addition, a method of causing all PCS that are linked to high-voltage distribution lines to which is connected a low-voltage distribution line in which voltage deviation has occurred to supply reactive power is not capable of resolving the inequalities among consumers because voltage suppression control does not have to be implemented in PCS that lack Q control capability. In addition, this method is not capable of effectively suppressing a voltage increase because the total voltage adjustment amount required to suppress the voltage resulting from each distributed generator is great.
Still further, a method in which losses that result from inequalities of the amounts of voltage adjustment are compensated for by money suffers from the problem of the difficulty of determining incentives that can resolve inequalities.